It is known to develop a latent electrostatic image with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are &lt;30 .mu.m determined by Malvern 3600E Particle Sizer described below. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
There are many methods of making liquid toners. In one method of preparation of the improved toner particles are prepared by dissolving at an elevated temperature one or more polymers in a nonpolar dispersant, together with particles of a pigment, e.g., carbon black. The solution is cooled slowly, while stirring, whereby precipitation of particles occurs. It has found that by repeating the above process some material was observed that was greater than 1 mm in size. By increasing the ratio of solids to nonpolar liquid the toner particles can be controlled within the desired size range, but it has been found that the density of images produced may be relatively low and when a transfer is made to a carrier sheet, for example, the amount of image transferred thereto may be relatively low. The particles in this process are formed by a precipitation mechanism and not grinding in the presence of particulate media and this contributes to the formation of an inferior toner.
In another method of preparation of toner particles, the plasticizing of the thermoplastic polymer and pigment with a nonpolar liquid forms a gel or solid mass which is shredded into pieces, more nonpolar liquid is added, the pieces are wet-ground into particles, and grinding is continued which is believed to pull the particles apart to form fibers extending therefrom. While this process is useful in preparing improved toners, it requires long cycle times and excessive material handling, i.e., several pieces of equipment are used.
In yet another method of preparation of toner particles for electrostatic imaging, the following steps are followed: A. dispersing at an elevated temperature in a vessel a thermoplastic resin, a nonpolar liquid having a Kauri-butanol value of less than 30, and optionally a colorant, at a total % solids of less than 18% by weight by means of moving particulate media whereby the moving particulate media creates shear and/or impact, while maintaining the temperature in the vessel at a temperature sufficient to plasticize and liquify the resin and below that at which the nonpolar liquid boils and the resin and/or colorant, if present, decomposes, B. cooling the dispersion to permit precipitation of the resin out of the dispersant, the particulate media being maintained in continuous movement during and subsequent to cooling whereby the toner particles are &lt;30 .mu.m determined by Malvern 3600E Particle Sizer described below and a plurality of fibers are formed, and
C. separating the dispersion of toner particles from the particulate media. This method can provide toners with a particle size of 10 .mu.m or less as determined by Malvern 3600E Particle Sizer but requires extremely long grinding times to achieve this desired particle size.
It has been found that the above disadvantages can be overcome and toner particles prepared by a process that does not require excessive handling of toner ingredients at elevated temperatures whereby toner particles having an average size (by area) of 10 .mu.m or less determined by Malvern 3600E Particle Sizer are dispersed and formed in the same vessel with greatly reduced grinding times. Transfer of an image of the so prepared toner particles to a carrier sheet results in transfer of a substantial amount of the image providing a suitably dense copy or reproduction.